The work during the initial CORT funding period established the following principles: i) periosteal MSCs respond to injury with proliferation and expansion; ii) The expanded population of MSCs secretes growth factors and responds to signals that drive bone repair; iii) COX-2/PGE2 is a key early reparative signal; iv) aging results in reduced COX-2 expression during bone repair; and 5) PTH{1-34} (PTH) compensates for the reduced healing observed in aging and COX-2[-/-] mice. The ongoing goal of Project 2 is to provide a clear and comprehensive paradigm of the cell populations and signaling mechanisms regulated by PTH during bone repair responsible for its anabolic effects. Aims 1 and 2 use in vivo models of bone repair, while Aim 3 uses a complementary in vivo approach. Experiments in Aim 1 define periosteum as a key target for bone repair with age dependent responsiveness. The experiments will show that PTH acts specifically on periosteal MSC and stimulates three signaling pathways that are potent mediators of bone formation: Wnt/p- catenin, BMP/Smad, and lhh/Gli-2/Ptc. Aim 2 provides genetic evidence that Wnt/Beta-catenin signaling is an essential component of the mechanism whereby PTH stimulates bone repair. Aim 3 uses a highly innovative approach to isolate and culture periosteal stem cells. Using genetically altered mice the signals that are sequentially activated downstream of PTH are examined in isolated periosteal stem cells. Collectively, the experiments in the 3 Aims will establish that 1) PTH enhances Wnt/p-catenin and BMP/Smad signaling (stem cell proliferation/expansion, osteoblast differentiation) from periosteal/endosteal MSCs); ii) Wnt/p-catenin stimulates BMP-2 expression and BMP/Smad signaling (osteoblast differentiation/bone formation and chondrogenesis); c) BMP/Smad and Wnt/Beta-catenin signaling stimulate Ihh/Ptc signaling (bone formation). The CORT Program is based upon the unifying concept that i) expansion and differentiation of the MSC population as a critical event in musculoskeletal repair; and ii) PTH is key anabolic agent that promotes tissue specific MSC expansion and repair events. The integration of Project 2 with the other CORT Projects through an Administrative Core that enhances collaborations and access to an innovative and comprehensive Molecular and Anatomic Imaging Core will leverage the remarkable progress of the initial funding period and lead to important novel insights and therapies for orthopaedic trauma patients.